1. Field of the Invention
The present invention relates to a rotor magnet that is configured to be mounted in a spindle motor, a spindle motor comprising the same, a recording and reproducing apparatus, and a jig for manufacturing the same.
2. Description of the Related Art
With the trend of reduction in size and weight of machines that a device such as a hard disk drive (HDD), a magneto-optical (MO) disk drive, an optical disk drive, and a floppy disk drive (FDD) is mounted therein, a variety of demands, such as increase in capacity and reduction in size, thickness, and cost, have been recently increasing for the machines.
For example, Japanese Patent Application Publication No. JP-A-2004-328978 (published on Nov. 18, 2004) discloses a brushless motor that is capable of being formed in a small size and is capable of reducing electric power consumption. Here, the brushless motor has a configuration that a ring made of a magnetic substance is attached to an end of a rotor magnet on a base plate side. According to the configuration, it is possible to reduce leakage magnetic flux from the rotor magnet to the base plate side. Therefore, even when the rotor magnet and the base plate are disposed adjacent to each other, it is possible to avoid an increase of load and inhibit electric power consumption. It is also possible to form a low-profile device with this configuration.
Also, Japanese Patent Application Publication No. JP-A-2003-274602 (published on Sept. 26, 2003) discloses a spindle motor that is provided with a magnetic attraction plate. Here, the magnetic attraction plate includes an axially opposed portion that is opposed to a rotor magnet in an axial direction, and a radially opposed portion that is opposed to the rotor magnet in a radius direction. According to the configuration, it is possible to efficiently use the magnetic flux from the rotor magnet as the magnetic attraction by means of both opposed portions. Accordingly, even when a simple and small spindle motor is formed, it is possible to efficiently secure the magnetic attraction by the magnetic attraction plate with respect to a rotational body.
Also, as an example of a method of magnetizing a rotor magnet, Japanese Utility Model Application Publication No. JP-U-H01-76007 (published on May 23, 1989) discloses a method of magnetizing a rotor magnet from the inner peripheral side thereof. In a normal magnetizing process, a magnetizing yoke is inserted into a rotor magnet, and the outer periphery of the magnetizing yoke engages with the inner periphery of the rotor magnet. Next, a yoke member is attached to and engages with the outer periphery of the rotor magnet.
Here, the yoke member is made of ring-shaped magnetic material and is formed to have constant inner and outer diameters. Next, pulse current is applied to a coil of the magnetizing yoke, and thus the rotor magnet is magnetized. The main body of the rotor magnet that is magnetized as described above is kept in an approximately uniformly-magnetized condition in a rotational axis direction.
Furthermore, Japan Patent Application Publication JP-A-2004-248461 discloses a spindle motor having a configuration that the diameter of an end portion of a rotor magnet is formed to be less than that of a center portion thereof and accordingly a gap between the rotor magnet and a stator core is formed to be gradually greater toward the end portion of rotor magnet, and a configuration that an end of a rotor magnet is weakly magnetized with a magnetizing yoke formed to have height less the length of the rotor magnet in a rotational axis direction. According to the configuration, the amount of magnetization in the end portion of the rotor magnet in the rotational axis direction is smaller than that of the center portion thereof. Therefore, it is possible to reduce the magnetic attraction that will be a cause of vibration when a minute misalignment is produced between the stator core and the rotor magnet in the axial direction. Accordingly, it is possible to reduce vibration and noise, which result from inhibition of electric power consumption and magnetic attraction.
However, the above described conventional motors have the following problems.
Specifically, in the motors disclosed in the above described publication Nos. JP-A-2004-328978 and JP-A-2003-274602, with the trend of a low-profile motor, it is getting difficult to secure an axial space to which members (e.g., a ring and a magnetic plate) for adjusting attraction of a magnet are attached. Thus, these members might become a factor in preventing production of a low-profile motor. In addition, it is required to attach members (e.g., a ring and a magnetic plate) for adjusting attraction of a magnet to a motor. Therefore, a manufacturing process might become complicated and the manufacturing cost might be increased.
Also, according to the rotor magnet of the spindle motor disclosed in the Japan Patent Application Publication JP-A-2004-248461, the amount of magnetization in the end portion of the rotor magnet in the rotational axis direction becomes smaller than that in the center portion thereof. Because of this, when diameter of the end portion of the rotor magnet is formed to be smaller than that of the center portion thereof, a large gap is produced between the stator core and the end portion of the rotor magnet in the rotational axis direction. Accordingly, magnetic resistance is increased and thus torque to be generated in the motor is reduced, and electric current consumption is increased. On the other hand, when a magnetizing yoke with length (i.e., height) shorter (i.e., lower) than length (i.e, height) of the rotor magnet in the rotational axis direction is used, magnetic field in magnetization will be unstably generated in connections of the magnetizing coil, which is disposed on both end portions of the magnetizing yoke in the rotational axis direction. Accordingly, marked variation is generated in magnetization and thus vibration and noise are increasingly produced. Furthermore, there has been a problem that the energy product of the rotor magnet is not efficiently exerted because of the incomplete magnetization in the both end portions of the rotor magnet in the axial direction, and electric current consumption is increased by the reduction of the torque to be generated in the motor.
FIGS. 31 and 32 illustrate a method of magnetizing a rotor magnet 507 that is configured to be used in an inner rotor type spindle motor in which a stator core is disposed on the outer periphery of a rotor magnet. In this case, the rotor magnet is generally magnetized in a condition that a magnetizing yoke 512 having a magnetizing coil 513 wound therearound is disposed on the outer periphery of the rotor magnet 507. Note that a stator core is disposed on the outer periphery of the rotor magnet 507 when mounted in a motor. Here, as illustrated in FIGS. 30, 33A and 33B, when the rotor magnet 507 is magnetized from the outer peripheral side with the magnetizing yoke 512 that has length (i.e., height) longer (i.e., larger) than length (i.e., height) of the rotor magnet 507 in the rotational axis direction for the purpose of effectively magnetizing the end portion of the rotor magnet 507, the rotor magnet 507 is magnetized in a externally expanding direction with respect to a stator core 508. Accordingly, attraction is increased on the bottom end portion, and leakage magnetic flux is increased on the top end portion.
An object of the present invention is to provide a rotor magnet that is capable of adjusting attraction of a magnet to be mounted in a spindle motor, a spindle motor comprising the same, a recording and reproducing apparatus, and a jig for manufacturing the same, with a simple configuration.